There's a few factors, all of which are in some level of change right now.
1) Cable providers in major markets (urban ones) are offering gbps service and often slightly faster (1.2gbps) now, at rates that I would say aren't great but are not terrible, in my case about $100 a month. Unfortunately that gigabit is downstream only. So high DOCSIS speeds are happening, just not as universally as you'd hope.
2) One of our biggest cable giants, Comcast, has a legacy channel planning problem related to the older Motorola STBs guide and on-demand delivery that prevents them reallocating additional channels to DOCSIS 3.1 upstream for the time being (those channels are in use by the Motorola STBs which don't support anything else). This results in a hard 35mbps upstream limit for most Comcast customers. Unfortunately the route to fixing this is to replace all of the older STBs, which Comcast is slowly doing but this requires getting customers to turn in their old one and pick up a new one... you can imagine how hard it is to get everyone to do this, even offering incentives.
3) DOCSIS 3.1 can achieve gbps+ speeds over an admirably long range but not for a very high customer count. So cable providers are transitioning their HFC network over time to a "node+0" architecture that resembles DSL with serving area DSLAMs... basically they are eliminating distribution amplifiers from the cable network and replacing them with compact field nodes (CMTS and head end) using fiber uplink. This essentially becomes a "fiber to the curb" architecture with cable last mile (more like last thousand feet in urban areas). It needs to be done mainly to reduce the number of customers on each CMTS to reduce contention, and sort of as a requirement for full performance of the DOCSIS 3.1 high speed upstream channels, but as you can imagine this large-scale change to outdoor plant is expensive and time-consuming, although it seems largely complete in a lot of cities.
4) As you get customers on very fast service plans backhaul at the node becomes a problem. This is mostly being addressed through installing new fiber backhaul routes for the new nodes, but in more suburban areas can be a big limitation.
5) All the TV on the cable takes up a lot of bandwidth and makes channel planning more difficult even with the abstraction in STBs. This is why major cable providers are starting to shift towards an IP-only "over the top" architecture where they deliver IPTV over DOCSIS instead of "real" cable television. This will help but once again requires all new STBs.